Containers and packaging materials such as for foods, toiletry products, drugs, medical supplies, electronic components, etc., should have strength and gas barrier properties sufficient to protect the contents. Most of currently employed gas barrier materials are occupied by those prepared from chlorine-based materials such as polyvinylidene chloride and those prepared by evaporation of inorganic substances. Hence, enormous amounts of carbon dioxide and heat are discharged during the course of the preparation and disposal. Further, with the chlorine-based materials, a problem is involved in the generation of dioxins. As to the evaporated films of inorganic substances, there have arisen some problems such as of damaging incinerators upon burning and requiring removal of films for recycling. Therefore, conversion of these gas barrier materials to eco-friendly materials has been being in progress.
A noteworthy eco-friendly material is cellulose. Cellulose is contained in cell walls of plants, excocrine secretions from microbes, mantles of sea squirts, etc., and is the most common polysaccharide on earth. Cellulose has biodegradability, high crystallinity and excellent stability and safety. Therefore, the development of application to various fields has been expected.
Because of strong intramolecular hydrogen bonds and high crystallinity, cellulose is almost insoluble in water and ordinary solvents. Therefore, studies on the improvement of solubility have been made extensively. Cellulose has three hydroxyl groups. When an oxidation reaction with a TEMPO catalyst system is performed, only the primary hydroxyl group at the C6 position of cellulose can be selectively oxidized and converted to a carboxyl group through an aldehyde group. Additionally, the reaction is feasible under mild conditions such as of an aqueous system or room temperature, for which much attention has been paid to cellulose recently. It will be noted that TEMPO is an abbreviation of 2,2,6,6-tetramethylpiperidine-1-oxyl.
It is known that TEMPO oxidation of natural cellulose enables only the nano-order crystal surface to be oxidized while keeping the crystallinity of cellulose and that mere addition of a slight mechanical treatment enables fine cellulose to be dispersed in water. It is also known that fine cellulose has high strength due to its high crystallinity and low linear expansion coefficient and that a film formed by drying the water-dispersed fine cellulose is gas impermeable.
PTL 1 has set forth a gas barrier laminate wherein a cellulose fiber layer containing fine cellulose fibers is stacked on a surface of a base material. In this gas barrier laminate, a suspension containing fine cellulose fibers is fed and attached to the surface of the base material thereby forming the cellulose fiber layer.